Breast cancer is the most common malignant neoplasm and the leading cause of cancer deaths among women in the United States. Early detection is critical to the successful treatment of this disease. Currently the most common methods of detections are mammography and ultrasound. Mammography is considered the only reliable means of detecting nonpalpable breast cancers. A drawback of mammography is that it uses ionizing radiation. Also, breast tissue can be radiologically dense, making detection difficult. Ultrasound is used primarily as either a first screening tool or as a supplement to mammography.
Other techniques for the detection of breast cancer are under investigation. Magnetic Resonance Imaging (MRI) is a technique which is superior to mammography in that it can distinguish solid lesions from cystic lesions. However, MRI is expensive, has inferior spatial resolution as compared to mammography and can not image micro calcifications. Breast cancer tomography has been investigated but is expensive, has poor spatial resolution and involves the use of intravenous injected iodinated materials.
A relatively new and active field is the use of non-ionizing laser light to detect breast cancer. The optical properties of normal and diseased breast tissue typically varies. Therefore, it is possible to detect breast cancers based on the optical differences of the tissue. This is due to the fact that cancerous tissues manifest a significant change at the cellular and sub-cellular level. For example, the scattering coefficient of fibrocystic tissue (600 cm.sup.-1) is approximately 50 percent higher than that of normal glandular breast tissue (400 cm.sup.-1) or 100% higher than normal breast adipose tissue (300 cm.sup.-1) in the wavelength of 500-1000 nm.
One laser technique is called "early photon imaging." Since breast tissue is an optically turbid medium, light is quickly diffused inside tissue as a result of scattering. Light in tissue takes one of three forms: ballistic light, which travels straight through tissue without scattering; quasi-ballistic light, which experiences some scattering; and diffuse light, which is almost completely scattered inside tissue. This technique uses pulses of light and attempts to detect only the first light that is transmitted through the tissue. Because early photon imaging only detects ballistic light, this technique is mainly useful for thin tissues. Diffuse light is needed to detect thick tissues (5 cm).
In order to increase the incidence of early detection of breast cancer, it is desirable to have a system which can detect small abnormalities with good resolution at low cost and without the use of ionizing radiation.